For part of an exam question, "why do cells grown in liquid cause turbidity?" I answered that cells refract light. The correct answer was that cells scatter light. Isn't the light scattering ultimately caused by lots of refraction events? Is the grader correct to take off full points?
You're both correct. Turbidity is causes by excessive light reflection events from all of the particles, and is actually how it is measured in a sample. As to cells, they both both reflect light that is incident on a membrane, but they will also refract light that does manage to enter the cell because of the difference in refractive indices. A portion of the light will also be absorbed by the cells.